One religious woman is performing harmoniously (2,6)
I believe the answer is:
in unison
'harmoniously' is the definition.
(thesaurus)
'one religious woman is performing' is the wordplay.
'one' becomes 'i' (Roman numeral).
'religious woman' becomes 'nun' (I've seen this before).
'performing' becomes 'on' (I've seen this in other clues).
'i'+'nun'+'is'+'on'='IN UNISON'
(Other definitions for in unison that I've seen before include "All speaking or singing at the same time" , "All singing the same notes" , "All together at the same pitch" , "All speaking or singing as one" , "Simultaneously" .)